In general, a fuel cell is a power generation system that directly transforms chemical energy due to reaction between hydrogen and oxygen into electrical energy. Here, a fuel cell system can either receive pure hydrogen or hydrogen obtained by reforming methanol, ethanol, natural gas or the like. Also, the fuel cell system can either receive pure oxygen or oxygen contained in air.
A polymer electrolyte membrane fuel cell (PEMFC), which operates at a temperature of 100° C. and below, includes a fuel container to store fuel, a reformer to generate hydrogen by reforming the fuel, and an electricity generator to generate electricity having certain voltage and current by electrochemical reaction between hydrogen and oxygen. The electricity generator includes at least one unit fuel cell to generate the electricity, and may have a structure in which a plurality of unit fuel cells are stacked.
In the PEMFC with this structure, the fuel stored in the fuel container is supplied to the reformer, the reformer generates hydrogen by reforming the fuel, and the electricity generator generates the electricity energy by the electrochemical reaction between hydrogen and oxygen.
The electricity generator substantially includes several to tens of unit fuel cells which are stacked. Here, each unit fuel cell includes a membrane-electrode assembly (MEA) and a bipolar plate. The membrane-electrode assembly includes an electrolyte membrane, and an anode and a cathode attached to opposite sides of the electrolyte membrane. The bipolar plate is used as not only a passage through which hydrogen and oxygen needed for the reaction are supplied, but also a conductor connecting the anode and the cathode of each MEA in series. Through the bipolar plate, hydrogen is supplied to the anode, and oxygen is supplied to the cathode. At this time, hydrogen is oxidized in the anode and oxygen is reduced in the cathode, thereby causing electrons to move and generating electricity.
The reformer applies a reforming reaction to a mixture of fuel and water, thereby not only producing a reformed gas containing rich hydrogen needed for generating electricity in the electricity generator, but also removing carbon monoxide from the reformed gas in order to protect a catalyst of the fuel cell from being poisoned by carbon monoxide. Such a reformer generally includes a reforming unit for reforming the fuel to generate the reformed gas containing rich hydrogen, and a carbon monoxide remover to remove carbon monoxide from the reformed gas. The reforming unit employs a catalytic reaction such as steam reforming (SR), partial oxidation (PDX), auto-thermal reforming (ATR), and the like to get the reformed gas containing rich hydrogen. The carbon monoxide remover employs a catalytic reaction such as water gas shift (WGS), preferential CO oxidation (PROX) and the like, or hydrogen refinement using a separation film in order to remove carbon monoxide from the reformed gas.
Various materials containing hydrogen can be used as the fuel for the PEMFC. Particularly, a hydro-carbonaceous material is generally used. As the hydro-carbonaceous material, butane has recently attracted attention because it is readily available and also employed as a combustion fuel for a heat source needed for the reforming reaction.
Butane has a low boiling point and is easily liquefied at a low pressure, so that it is contained in the fuel container as being liquefied by a predetermined pressure. In the case of the fuel cell system employing butane contained in the fuel container, gasified fuel is discharged of itself from the fuel container to the fuel cell when a nozzle of the fuel container is opened, so that an additional fuel-supplying unit is not needed. However, because other water-supplying unit is still needed to drive the fuel cell, removing only the fuel-supplying unit so as to decrease the volume and the power consumption of the fuel cell system serves little purpose.